Interface - The Journal of Wheel/Rail Interaction

GIRDER RAIL

Testing Girder Rail on the MBTA (continued)

Matching Flangeways
The flangeway width of the Bethlehem 149-pound rail was a little more than 1-13/16 inches, the prevailing standard for more than 50 years on the MBTA. As a result, the flangeway width at all tongue and mate street switch castings was also a little more than 1-13/16 inches. This created a compromise problem when the MBTA decided to transition to the use of GGR-118 rail, which had a flangeway width of a little less than 1-5/8 inches. This meant that where it was necessary to use the new rail to tie into existing street switches, there would be a flangeway discrepancy of about 1⁄4 inches. The restraining face of the switch casting arms was welded to make the flangeway width compatible.

The flangeway of the NP4aM rail was another story altogether. It was slightly less than 1-3/8 inches — about 1/4-inch less than the GGR-118 and 1/2-inch less than the original Bethlehem 149-pound rail. This was a significant issue. Aside from the compromise issues, the MBTA was concerned that the flangeway might be too narrow for MBTA light rail vehicle wheel flanges, especially in sharp curves.

The flangeway situation was further complicated by the fact that the MBTA had recently transitioned to a new wheel profile on its light rail vehicles. After having run the same wheel profile on several generations of streetcars, trolleys and light rail vehicles, the MBTA introduced a markedly different profile in 2002 - 2003. This wheel profile, known as the Interim Wheel Profile (or IWP), was designed to combat a wheel climb problem on the independently rotating wheels of the low-floor center section of new light rail vehicles that were delivered in 1999 - 2000.

The major modification to the wheel profile was a change in the flange angle, which increased to 75 degrees from the longstanding standard of 63 degrees. The flange on the new wheel profile was also thicker. The prospect of introducing the new NP4aM girder rail profile with a flangeway width that was more than 25% narrower than the previous section, coupled with a new wheel profile with a steeper flange angle and thicker flange was cause for concern among MBTA engineers and their consultants.

Investigation
In railway engineering circles, one of the most significant studies of the dynamic representation of wheel flanges in curved track was the Filkins-Wharton diagrams. These were manual calculations which incorporated wheel flange, angle of attack and wheel skew data relative to various curve radii to develop wheel flange “clearance envelopes.” With these diagrams, it was possible to represent what the wheel flange actually looks like in a curve in the top of rail plane. The Filkins-Wharton diagrams were an important tool in track design for many years. They were, however, somewhat difficult to interpret and use.

When the MBTA decided to investigate the use of NP4aM girder rail as its standard rail section for street running track, John I. Williams, an independent consultant working for HNTB Rail Systems, took a creative approach. Using “Solid-Works” software, Williams produced Filkins-Wharton diagrams to develop a dynamic representation of the MBTA’s new wheel profile in a 50-foot radius curve.

Because a wheelset is not radial when in a curve, a determination of the angle of attack had to be made for use with the Solid-Works program. Calculations showed that the angle of attack due to curvature in a 50-foot radius curve for the MBTA’s light rail vehicle would be about four degrees. This calculation included an allowance for skewing of the vehicle truck due to “crabbing.” The wheel flange skewed at an angle of attack of about four degrees made the wheel flange “clearance envelope” around 1/4-inch wider than the actual wheel flange at a point 0.25 inches below the wheel baseline (see Figure 2).

The information developed by the Williams team was critical to the MBTA in its investigation of the NP4aM girder rail. Conventional wisdom had always dictated that gauge-widening, especially in tight radius curves constructed with girder guard rail, was necessary to allow free passage of vehicle trucks. The practice at the MBTA had been to increase standard gauge by 1/4-inch (to 56-3/4 inches) in single restrained girder guard rail curves (100-foot – 1000-foot radius) and to increase the gauge by as much as 3/4 inches (to 57-1/4 inches) in double restrained girder guard rail curves (<100-foot radius).

Experience, however, demonstrated that widening gauge on tight radius curves actually led to excessive skewing of the vehicle trucks and a disproportionate amount of rail wear at the restraining rail faces of both the inside and outside rails. The same disproportionate wear was likely taking place on diagonally opposite wheels on the light rail vehicles.

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